The present invention relates to reactive power compensators of the general type described in German Pat. No. 2,303,939. In the circuit of this patent, a compensation capacitor is precharged to the peak value of the network alternating voltage during a quarter period of the alternating voltage. The capacitor is connected by means of a thyristor switch, and via a choke, to the alternating current network to be compensated when the voltage at the thyristor switch is approximately 0. Within a quarter period after the switch-off time of the capacitor, the latter is discharged by the firing of the thyristor switch.
If in such an arrangement the thyristor switch is closed, i.e. electrically conducting, and the capacitor is thus connected to the alternating voltage network, the voltage across the capacitor corresponds at any instant to the network voltage. If the capacitor is disconnected from the alternating voltage network by opening the thyristor switch, the voltage at one terminal of the thyristor switch is the same as that of the capacitor at the time of switching. Subsequently, as the capacitor voltage and the network voltage change, the voltage difference across the thyristor changes.
The capacitor voltage decreases in time, at least in the mean, by being discharged via a discharge circuit connected in parallel with it. As a rule, the capacitor is disconnected from the alternating voltage network at the zero transition of the current, that is to say at the time of maximum network voltage. If the discharging of the capacitor via the discharge circuit is a slow process in comparison with the duration of a cycle of the alternating voltage, practically twice the maximum network voltage is present across the thyristor switch after half a cycle's duration. In order that the thyristor switch need be dimensioned only for a single maximum mains voltage, which is of considerable advantage for economic reasons, the capacitor must be capable of being discharged fast enough via its discharge circuit, at most over half a period of the alternating voltage. The duration of half a period is 10 ms with a 50 Hz frequency of the alternating voltage. The capacitor customarily has a capacity of the order of magnitude of some 100 .mu.F. In order for such a large capacitance to be discharged at all in 10 ms, the discharge circuit must have a low impedance. A purely ohmic resistance in the discharge circuit should be, for example, only a few ohms, which practically represents a short circuit for the capacitor, with a correspondingly high power loss. Such a loss cannot be tolerated when the capacitor is connected to the alternating voltage mains. This power loss can be limited by a switch which is arranged in the discharge circuit and which is preferably a fast-acting semiconductor switch that is open during the time the capacitor is connected to the alternating voltage mains and is closed only when the capacitor is disconnected from the alternating voltage mains for initiation of the discharge process. Such a switch must be dimensioned for the full maximum mains voltage and, therefore, also involves high costs.
In accordance with the previously mentioned German Pat. No. 2,303,939, the capacitor can be discharged via a transformer into an auxiliary voltage network. Together with the inductance of a choke arranged in the discharge circuit, the inductance of the primary winding of the transformer, which is also arranged in the discharge circuit, and the capacitor form a resonant circuit. This arrangement also requires a switch in the discharge circuit, by means of which switch a free compensating oscillation can be switched on at the time the capacitor is disconnected from the alternating voltage mains and can subsequently be interrupted.